Cut shapes off a surface
There are a bit of errors:


The pot-like object has to be converted to a mesh. (Alt+C then Select Mesh from Curve/Meta/Surf/Text)
The modifier should be applied to the pot-like object and the object in the modifier should be the parallelepiped (The one Selected in The Photo).
The normals in the pot-like object are pointing in the wrong
direcion, so you have to adjust them if you won't the modifier not
to have a strange behavior.  to adjust the normal press tab to go
into edit mode, then press w and select Flip Normals


Then The modifier Should Work.